1. Field of the Invention
The present invention relates to a fuel supply apparatus for an internal combustion engine having a fuel injection mechanism (in-cylinder injector) for injecting fuel into a cylinder at a high pressure, or for an internal combustion engine having, in addition to the above fuel injection mechanism, another fuel injection mechanism (intake manifold injector) for injecting fuel into an intake manifold or an intake port. More particularly, the present invention relates to a fuel supply apparatus for an internal combustion engine capable of preventing leakage of fuel from a high-pressure fuel system at the time of stop of the internal combustion engine and also capable of appropriately handling vapor generated within a fuel pipe.
2. Description of the Background Art
Generally, in an engine for an automobile, fuel is supplied from a fuel tank via a fuel pump and a fuel pipe to an engine (internal combustion engine), where the fuel is injected via an injector.
A direct injection engine having an in-cylinder injector for injecting fuel into a combustion chamber of a gasoline engine is known. Further, an engine having an in-cylinder injector for injecting fuel into a combustion chamber of a gasoline engine and an intake manifold injector for injecting fuel into an intake manifold, and performing fuel injection using both of the in-cylinder injector and the intake manifold injector in accordance with the engine speed or the load of the internal combustion engine is also known. In a high-pressure fuel system including the in-cylinder injector, the fuel having a fuel pressure increased by a high-pressure fuel pump is supplied via a delivery pipe to the in-cylinder injector, and the in-cylinder injector injects the high-pressure fuel into a combustion chamber of a corresponding cylinder in the internal combustion engine.
As such, a high-pressure fuel pump is used to make the fuel attain a high-pressure state in the internal combustion engine. In the high-pressure fuel pump, a cylinder is driven by means of a cam that is provided at a driveshaft connected to a crankshaft of the internal combustion engine.
In the internal combustion engine as described above, in order to improve restarting capability at a high temperature at the time of engine start, it is necessary to prevent occurrence of vapor in the fuel within the fuel pipe. Thus, in a conventional fuel injection control apparatus for an internal combustion engine, a check valve is provided at the discharge side of the fuel pump, and the fuel residual pressure within the fuel pipe is not lowered even while the engine is stopped, so as to maintain a high fuel pressure.
When the fuel pressure within the fuel pipe is maintained at a high pressure while the engine is stopped, however, there may occur leakage of the fuel from the injector into the intake pipe. The fuel pressure maintained at a high pressure during the time of engine stop is decreased to a level equivalent to the atmospheric pressure (=0.1 [MPa]) in about 60 minutes, during which the gasoline would leak in a quantity as large as about 20 mcc per fuel pipe.
Such fuel leakage causes an increase of unburned HC within the exhaust gas at the time of next engine start. The amount of HC emission at the time of start becomes very large within the time of about one second. Further, the quantity of the fuel leaked from the injector cannot be controlled, thus causing variation in exhaust gas components at the time of engine start.
Furthermore, the fuel leaked into the intake pipe may increase the fuel vapor gas emitted from the automobile. The situation is approaching an intolerable level, with the exhaust gas regulations having become increasingly stringent in recent years.
Japanese Patent Laying-Open No. 08-028382 discloses a fuel pressure control apparatus of a high-pressure injection type engine, that can control the high-pressure state of the fuel line in a stable state where no vapor occurs during the engine operation, and that can surely lower the pressure of the fuel line close to the atmospheric pressure level after the stop of the engine. This fuel pressure control apparatus of a high-pressure injection type engine is provided with a fuel pump for high pressure on a fuel line, and is also provided with a pressure regulator for high pressure that is arranged downstream of the fuel pump for high pressure. A high-pressure line is arranged between the fuel pump for high pressure and the pressure regulator for high pressure, a low-pressure delivery line is arranged from the fuel tank to the upstream of the fuel pump for high pressure, and a low-pressure return line is arranged from the downstream of the pressure regulator for high pressure to the fuel tank. An injector for injecting fuel directly into a combustion chamber is connected to the high-pressure line. In the high-pressure injection type engine configured as described above, the pressure regulator for high pressure is of a constantly open type, which is in an open state when the engine is stopped. On the low-pressure return line arranged downstream of the pressure regulator for high pressure, a mechanical type pressure regulator for low pressure and a constantly open type valve that is closed at the time of engine operation, are arranged in parallel.
According to this fuel pressure control apparatus for a high-pressure injection type engine, the fuel pump for high pressure operates during the engine operation, to narrow down the pressure regulator for high pressure arranged downstream of the fuel pump for high pressure on the fuel line, so as to adjust the fuel pressure. Thus, the fuel pressure in the high-pressure line between the fuel pump for high pressure and the constantly open type pressure regulator for high pressure is maintained at a high pressure. The constantly open type valve connected in parallel with the mechanical type pressure regulator for low pressure, which are arranged on the low-pressure return line connecting the downstream side of the pressure regulator for high pressure and the fuel tank, is maintained at a closed state during the engine operation. Thus, the relief fuel from the pressure regulator for high pressure has its pressure adjusted by the mechanical type pressure regulator for low pressure, and the fuel is returned to the fuel tank while the fuel pressure is gradually reduced instead of being rapidly reduced. Meanwhile, when the engine is stopped with the ignition switch OFF, the constantly open type pressure regulator for high pressure is opened, and the constantly open type valve is opened. As a result, the fuel pressure on the high-pressure line is released to the fuel tank from the constantly open type pressure regulator for high pressure via the constantly open type valve. In this manner, the pressure regulator for high pressure is opened when the engine is stopped, and the fuel pressure on the high-pressure line is surely reduced close to the level of the atmospheric pressure, and accordingly, leakage of the fuel from the injector in communication with the high-pressure line can be prevented.
When the fuel pressure of the high-pressure line is decreased after the engine stop as described in Japanese Patent Laying-Open No. 08-028382, however, the following problem will arise. Since the fuel pressure during the engine operation is very high, the decrease of the fuel pressure will cause boiling under reduced pressure, so that the air may be generated within the delivery pipe. The air may enter the delivery pipe from the gap of the relief valve as well. If the air thus occurred in the delivery pipe is injected from the injector into the combustion chamber, the air-fuel ratio may be disturbed (to the lean side) at the time of next start of the internal combustion engine. The extremely lean state may even cause misfire.